1. Field of the Invention
This invention relates to high frequency electronic circuits, and more particularly to microwave communication filters implemented using planar transmission line fabrication techniques.
2. Description of Background Art
Design techniques for single mode planar microwave filters, such as broadside edge coupled filters, have long been established. Implementation of planar microwave filters is often achieved using microstrip and stripline fabrication techniques. Microstrip is formed by etching a circuit pattern on one side of two metal layers separated by a dielectric substrate. The unetched side serves as a ground plane. Stripline circuits are fabricated by etching a metal layer sandwiched between two dielectric layers having outer surfaces coated by metal ground planes. These single mode planar filters, however, are of limited utility for most high performance microwave applications due to their typically high insertion loss and their impracticality for filter passbands of less than 5%. The high performance requirements for communication satellite frequency multiplexers typically require the use of dual mode cavity or dielectric resonator filters to realize self equalized, quasi-elliptic responses having pass bands often less than 1%. These filters have the drawbacks of relatively large size and high cost.
In U.S. Pat. No. 3,796,970 by Snell, an orthogonal resonant filter was disclosed in which the two surface dimensions are each designed to be one-half the wavelength of a desired frequency. FIG. 1 shows the resonator 2 of Snell having a rectangular shape with side lengths of 1.sub.1 and 1.sub.2. Signal conductors 4 are used to couple signals to and from resonator 2. Accordingly, the element supports two resonant orthogonal standing waves, and external coupling to each wave can be provided independently.
In Soviet Union patent no. 1,062,809, a rectangular resonator is shown with inputs and outputs electromagnetically coupled to the resonator.
In Japanese patent no. 58-99002, an adjustable notch in a slot line ring is disclosed for tuning the center frequency and bandwidth of a microwave filter.